[PATCH AUTOSEL 5.5 05/58] sched/fair: Prevent unlimited runtime on throttled group

[Sasha Levin]

From: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

[ Upstream commit 2a4b03ffc69f2dedc6388e9a6438b5f4c133a40d ]

When a running task is moved on a throttled task group and there is no
other task enqueued on the CPU, the task can keep running using 100% CPU
whatever the allocated bandwidth for the group and although its cfs rq is
throttled. Furthermore, the group entity of the cfs_rq and its parents are
not enqueued but only set as curr on their respective cfs_rqs.

We have the following sequence:

sched_move_task
  -dequeue_task: dequeue task and group_entities.
  -put_prev_task: put task and group entities.
  -sched_change_group: move task to new group.
  -enqueue_task: enqueue only task but not group entities because cfs_rq is
    throttled.
  -set_next_task : set task and group_entities as current sched_entity of
    their cfs_rq.

Another impact is that the root cfs_rq runnable_load_avg at root rq stays
null because the group_entities are not enqueued. This situation will stay
the same until an "external" event triggers a reschedule. Let trigger it
immediately instead.

Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Acked-by: Ben Segall <bsegall@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/1579011236-31256-1-git-send-email-vincent.guittot@xxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 kernel/sched/core.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 1f4f8e850cab7..4b6b916ec1be4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7067,8 +7067,15 @@ void sched_move_task(struct task_struct *tsk)
 
 	if (queued)
 		enqueue_task(rq, tsk, queue_flags);
-	if (running)
+	if (running) {
 		set_next_task(rq, tsk);
+		/*
+		 * After changing group, the running task may have joined a
+		 * throttled one but it's still the running task. Trigger a
+		 * resched to make sure that task can still run.
+		 */
+		resched_curr(rq);
+	}
 
 	task_rq_unlock(rq, tsk, &rf);
 }
-- 
2.20.1